Deadly bombings hit the Daqahliya Security Directorate in the early hours of Tuesday morning in the Nile Delta city of Mansoura.
The blast killed 15 and injured 134 in what appears to be the worst terrorist attack on a government site since the ouster of Islamist president Mohamed Morsi last July.
Nine policemen, two civilian and three unidentified individuals were among the dead, Egypt's Interior Ministry said in a statement.
The explosion ripped through the building's side façade and damaged a number of police vehicles and parts of adjacent buildings which include the city's council building, a state-owned theatre and a bank.
The head of Mansoura's Security Directorate was among the injured.
Al-Ahram Arabic's correspondent in Mansoura said streets in the city of 500,000 residents became calmer after sunrise as thousands of citizens helping with rescue operations returned home once victims of the blast were transferred to area hospitals.
Thousands of citizens heeded official calls to donate blood in area hospitals.
However, doctors began turning away potential donors as blood banks acquired large quantities of blood.
The wreckage of an armoured police car could be seen nearby, while at least 10 civilian cars were damaged and a nearby building completely collapsed, said an AFP correspondent at the scene.
Furious reactions, denials
Angry residents of the area vented their fury on Morsi's Brotherhood movement."The Muslim Brotherhood is an international terrorist organisation. They are responsible for what happened in Mansoura," Hamada Arafat, a schoolteacher, told AFP.
"They are now adopting tactics like Al-Qaeda."
Prosecutors and officials say the Brotherhood has links with Islamist militants who have stepped up attacks on security forces across Egypt especially in the restive Sinai Peninsula.
"This is a more sophisticated attack than previous ones. It could be a sign of things to come... the insurgency in Sinai is becoming emboldened and extending attacks outside the Sinai," said Shadi Hamid, research director at the Brookings Doha Centre.
Egypt’s Interior Minister Mohamed Ibrahim, who visited victims of the blast in a Mansoura hospital, said the deadly attack came in retaliation to the violent dispersal by security forces of two Islamist protest camps in Cairo in August.
Ibrahim blamed the attack on the Muslim Brotherhood, saying the group sought to exact revenge from security forces, after two main Islamist protest camps were dispersed mid-August.
The Muslim Brotherhood was quick to deny any connection to the attack.
"The Muslim Brotherhood considers this act as a direct attack on the unity of the Egyptian people and demands an enquiry forthwith so that the perpetrators of this crime may be brought to justice," an emailed statement from the group's London press office read.
The pro-Morsi National Alliance to Support Legitimacy has also condemned what it labeled a "criminal incident" that aims to provoke strife, asserting its peacefulness and denouncing all forms of bloodshed, reported the Brotherhood's Freedom and Justice Party on its website.
Cause of blasts
Al-Ahram’s Arabic website quoted on Tuesday morning an anonymous security source as saying that two bombs had gone off almost simultaneously.
The first bomb, he said, was planted in a higher floor in the building, the second in a car next to the Security Directorate.
A third bomb planted in another car was defused, the source added.
However, Ministry of Interior statements have not specified the sources of the blasts.
A high-profile Ministry of Interior security source told MENA on Tuesday afternoon that primary information indicates a truck containing explosive materials was used in the blast.
The security source said a massive amount of high explosives was used in the incident, pointing out that experts are still examining the site of the blast to determine whether the vehicle had been remotely detonated or exploded through a timer.
Egypt's Prime Minister Hazem El-Beblawi vowed to hunt down the perpetrators of the explosion. He said the attack seeks to obstruct the roadmap drawn up by the country’s interim rulers following Morsi's July ouster by the army amid nationwide mass protests against his rule.
"This is an act of terrorism that aims at frightening the people and obstructing the roadmap. The black hands behind this act want to destroy the future of our country," Beblawi told Egyptian satellite channel ONTV.
"The state will do its utmost to pursue the criminals who executed, planned and supported this attack," he asserted.
Beblawi refused to confirm that Egypt had designated the Muslim Brotherhood a terrorist group, contradicting an earlier statement by one of his aides.
Cabinet Spokesman Sherif Shawqi had earlier blamed the blast on the Muslim Brotherhood group of ousted Islamist president Morsi, and said Prime Minister Hazem El-Beblawi had officially declared the group a terrorist organisation.
“Whoever is behind this act is a terrorist and will be brought to justice and punished according to the law. But I do not wish to anticipate events," Beblawi added.
The final stages of Egypt's roadmap will begin when a referendum on the newly-drafted constitution, scheduled for mid-January, takes place. It will be followed by parliamentary and presidential elections.
5 turbulent months
A substantial number of bombings have been executed across Egypt since the ouster of Islamist president Morsi.
On 24 July, an explosive device detonated at a police station near the Security Directorate in Mansoura, killing at least one conscript and injuring 19.
Egypt's Interior Minister Mohamed Ibrahim survived an assassination attempt on 5 September when a bomb exploded near his convoy in northeast Cairo.
However, most bomb attacks since Morsi's overthrow have occurred in the Sinai region, which borders Israel and the Palestinian Gaza Strip.
Approximately 200 soldiers have been killed in Sinai since July.
Numerous police stations and churches were also attacked across the country in the immediate aftermath of the forced dispersal of two pro-Morsi protest camps in Cairo on 14 August , which left hundreds dead.
Over 100 policemen have been killed in confrontations with militants in various parts of the country since then.
Ansar Beit Al-Maqdis, an Al-Qaeda-inspired Islamist militant group, has claimed responsibility for some of the attacks, including the attempt to assassinate the interior minister.
The militant group has warned the country's military and police, urging troops to desert their ranks or face death at the hands of its fighters.
The group said in a statement on militant websites Monday it considers Egyptian troops to be infidels because they answer to a "secular government."